The invention relates to a vehicle steering system having a steering grip control, in particular a steering wheel, which is activated by the driver, as well as a motor-operated actuator unit which is assigned to the steered wheels of the vehicle and to which the steering grip control is operatively connected via a controlled system, the controller of which is connected on the input side, for a desired value/actual value comparison, to a desired value transmitter which is activated by the steering grip control and to an actual value transmitter which registers the steering angle of the steered wheels of the vehicle.
At present, passenger cars and similar motor vehicles are as a rule equipped with hydraulic power-assisted steering systems in which the steering wheel is mechanically forcibly connected to the steered wheels of the vehicle, i.e. there is a mechanical drive connection between the steering wheel and steered wheels of the vehicle. Here, the steered wheels of the vehicle are also connected in terms of drive to a motor-operated actuator unit which is controlled as a function of the forces or moments transmitted between the steering wheel and steered wheels of the vehicle. For this purpose, components, for example shaft components, of the mechanical drive connection between the steering wheel and steered wheels of the vehicle are connected to one another in a sprung fashion so that the aforesaid components or shaft components execute a larger or smaller actuation travel or a larger or smaller rotation relative to one another depending on the respectively effective forces or moments. This actuation travel or this relative rotation then causes a servovalve which controls the hydraulic actuator unit to be activated.
As appropriate, an electroactuator unit can also be controlled by means of this actuation travel or the relative rotation.
In aeroplanes it is known to connect tail surfaces and wing flaps and the like to associated grip controls in a merely operative fashion via a controlled system, the grip control activating a desired value transmitter and the wing flaps or tail surfaces activating an actual value transmitter, and the controller of the controlled system processing the signals of the desired value transmitter and actual value transmitter in terms of a desired value/actual value comparison and controlling motor-operated actuator units for the tail surfaces or wing flaps as a function of the result of this comparison. This concept, which is also referred to as "fly by wire", has in the meantime become so reliable that even passenger aeroplanes are equipped in this way.
In principle, it is also possible to provide comparable arrangements in vehicle steering systems, as is shown in EP 05 39 823 A1. In the said publication, the mechanical drive connection between the steering grip control and steered wheels of the vehicle is, however, not completely dispensed with. Instead, there is merely provision for the mechanical drive connection to be deactivated when the controlled system is operating without faults. As a result, it is possible, during normal operation, to realize parameter-dependent steering kinematics which differ from the mechanical drive connection.
The object of the invention is, then, to specify an advantageous arrangement for a steering system of the type mentioned at the beginning.
This object is achieved according to the invention in that a hydraulic actuator unit, and to activate it, two separate hydraulic circuits with in each case their own control valve unit which differs structurally from the corresponding unit of the other hydraulic circuit, are provided and in that a safety valve arrangement can be switched over between a normal state, which can be switched on by means of motor-operated actuator drives without self-locking features and in which the one hydraulic circuit is active, and an emergency state, which can be switched on by means of continuously present restoring force when the actuator drives are switched off and in which the other hydraulic circuit is active.
The invention is based on the general idea that, while a mechanical drive connection between the steering grip control and steering wheels of the vehicle is dispensed with, as is the possibility of connecting the steering grip control and steered wheels of the vehicle merely operatively via a controlled system for steering activation of the steered wheels of the vehicle, hydraulic systems are used whose essential system components have also been used in conventional power-assisted steering systems and have proven highly reliable in that context.
Furthermore, an--asymmetrical--redundancy is ensured in that hydraulic circuits which are arranged in duplicate are provided with different control valve arrangements which are independent of one another.
Expediently, the control valve units of the controller have electromagnetic actuator drives which are controlled on the output side and whose electric power supply is provided under normal circumstances by a main battery of the vehicle, which can also supply other electric loads of the vehicle. In addition, an emergency battery may be provided in order, if appropriate, to have a second electric power supply.
If hydraulic pumps which are arranged in duplicate are then also provided, it being possible for the one hydraulic pump to be driven by the vehicle engine and the other hydraulic pump to be driven electrically, and if, furthermore, the possibility of, if appropriate, also using system components of a hydraulic circuit in the other hydraulic circuit by interlinking the hydraulic circuits with one another by means of shut-off valves, it is possible overall to implement an extremely reliable vehicle steering system without the necessity for a mechanical drive connection between the steering grip control and steered wheels of the vehicle. In this way, at the same time, the advantage is obtained over vehicles with mechanical forcible coupling between the steering grip control and steering wheels of the vehicle that, in the event of an accident, there is an increased possibility that the steering system will remain operational and that it will not be possible for the steering grip control to be displaced toward the driver by elements of the mechanical drive connection.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.